1. Field of the Invention
The present invention relates to a liquid form addition curing type silicone rubber composition for forming rollers used in devices such as copiers, laser printers and facsimiles. Specifically, the present invention relates to a liquid form addition curing type silicone rubber composition for forming the roller used as a fixing roller, a developing roller or a paper supply roller in the above devices, and particularly to a liquid form addition curing type silicone rubber composition which displays good stability and undergoes almost no variation in viscosity at room temperature, and yet displays excellent curability at high temperature, as well as a roller comprising a layer of a silicone rubber elastic body formed from such a composition provided around the periphery of a metal core.
2. Description of the Prior Art
Liquid form addition curing type silicone rubber compositions are widely used for the fixing rollers, developing rollers and paper supply rollers used in devices such as copiers, laser printers and facsimiles. The reason for their use is that in comparison with other rubber materials, silicone rubber displays superior properties of releasability relative to toner, heat resistance and compression set. The formation of this type of roller usually involves either separating the constituents of the liquid form addition curing type silicone rubber composition into two parts, mixing them separately in a static mixer, and then combining the two parts at the time of use, or alternatively mixing all of the constituents together using the screw rotation action of a dynamic mixer. The thus obtained mixed product is then injected into a roll forming mold, cured under heat, and the cured roller then removed from the mold.
However, the time taken from preparation of the composition through to completion of the filling of the roll forming mold is considerably longer than typical injection molding, and because the mixed silicone rubber composition gradually begins to cure even at room temperature, problems such as thickening of the composition and gelling within the mold increase the likelihood of an unsatisfactory molding process. As a result, a composition which offers good stability and very little variation in viscosity over time at room temperature has been keenly sought. In contrast, once injected into the mold, the silicone rubber composition also needs to cure promptly under heating.
The present invention is the result of intensive research aimed at resolving the issues described above, and has an object of providing a liquid form addition curing type silicone rubber composition for roll forming purposes which displays little variation in viscosity at room temperature, and yet displays excellent curability at high temperature.
The inventors of the present invention discovered they could achieve this object with a liquid form addition curing type silicone rubber composition for roll forming purposes comprising:
(a) 100 parts by weight of an organopolysiloxane with at least two alkenyl groups bonded to non-terminal silicon atoms within the molecular chain of each molecule;
(b) 1 to 300 parts by weight of an inorganic filler;
(c) an effective quantity of a platinum based catalyst; and
(d) an organohydrogenpolysiloxane represented by a general formula (2), 
wherein, R3 represents an unsubstituted or a halogen-substituted monovalent hydrocarbon group with no aliphatic unsaturated double bonds, each R4 represents, independently, either an unsubstituted or a halogen-substituted monovalent hydrocarbon group with no aliphatic unsaturated double bonds or a hydrogen atom, and x represents an integer of 2 or greater,
with at least two silicon atom-bonded hydrogen atoms (in other words, SiH groups) within a single molecule, in a sufficient quantity to produce a molar ratio of silicon atom-bonded hydrogen atoms within the constituent (d) relative to the alkenyl groups bonded to silicon atoms within the constituent (a) of 0.1 to 5.
As follows is a more detailed description of the present invention. First, each of the constituents of a silicone rubber composition of the present invention are described.
Constituent (a): Organopolysiloxane
Constituent (a), which is one of the essential constituents of a composition of the present invention, is an organopolysiloxane with at least two alkenyl groups bonded to non-terminal silicon atoms within the molecular chain (in other words, alkenyl groups bonded to the silicon atoms of bifunctional siloxane units (i.e., diorganosiloxane units) or trifunctional siloxane units (i.e., organosilasesquioxane units) represented by the formulas R2SiO2/2 or RSiO3/2 (wherein R is an unsubstituted or halogen-substituted monovalent hydrocarbon group, as described below) to form so-called pendent substituent groups). The molecular structure of this organopolysiloxane should preferably be essentially a straight chain, although a molecular structure with some minor branching is also acceptable.
This organopolysiloxane should preferably be of a structure represented by a general formula (1) shown below. 
(wherein, R represents an unsubstituted or halogen-substituted monovalent hydrocarbon group, R1 represents an unsubstituted or a halogen-substituted monovalent hydrocarbon group with no aliphatic unsaturated double bonds, R2 represents an alkenyl group, n represents an integer of 2 or greater, and m represents 0, or an integer of 1 or greater).
Suitable examples of the unsubstituted or halogen-substituted monovalent hydrocarbon groups represented by R in the general formula (1) typically include hydrocarbon groups of 1 to 10, and preferably 1 to 6, carbon atoms, including alkyl groups such as methyl groups, ethyl groups, propyl groups, isopropyl groups, butyl groups, isobutyl groups, tert-butyl groups, pentyl groups, hexyl groups and cyclohexyl groups; alkenyl groups such as vinyl groups, allyl groups, propenyl groups and butenyl groups; aryl groups such as phenyl groups, tolyl groups and xylyl groups; aralkyl groups such as benzyl groups and phenylethyl groups; and halogenated alkyl groups such as 3-chloropropyl groups and 3,3,3-trifluoropropyl groups. Suitable R1 groups include the same unsubstituted or halogen-substituted monovalent hydrocarbon groups mentioned above but excluding those groups with aliphatic unsaturated double bonds such as the alkenyl groups including vinyl groups and allyl groups. Amongst the groups listed above, both the R groups and the R1 groups should preferably be methyl groups. The R2 groups represent alkenyl groups such as vinyl groups, allyl groups, propenyl groups and butenyl groups. n represents an integer of 2 or greater, and m represents either 0, or an integer of 1 or greater, and m+n is typically an integer of 20 to 2000, and preferably an integer of 50 to 1000. Furthermore, n/(m+n) should preferably be a number from 0.001 to 0.2, with values from 0.002 to 0.1 being particularly preferred.
The viscosity of the constituent (a) at 25xc2x0 C. should preferably be at least 1000 mPaxc2x7s (milliPascalxc2x7seconds), with values within a range from 1000 to 1,000,000 mPaxc2x7s being particularly preferred.
Specific examples of the constituent (a) include copolymers of dimethylsiloxane and methylvinylsiloxane with both terminals blocked with trimethylsiloxy groups, copolymers of dimethylsiloxane and methylvinylsiloxane with both terminals blocked with dimethylvinylsiloxy groups, copolymers of methylvinylsiloxane and diphenylsiloxane with both terminals blocked with trimethylsiloxy groups, copolymers of methylvinylsiloxane and diphenylsiloxane with both terminals blocked with dimethylvinylsiloxy groups, copolymers of dimethylsiloxane, methylvinylsiloxane and diphenylsiloxane with both terminals blocked with trimethylsilyl groups, copolymers of dimethylsiloxane, methylvinylsiloxane and diphenylsiloxane with both terminals blocked with dimethylvinylsiloxy groups, and copolymers of methylvinylsiloxane and methylphenylsiloxane with both terminals blocked with trimethylsilyl groups. These copolymers may be used singularly, or in combinations of two or more different copolymers.
Constituent (b): Inorganic Filler
The inorganic filler used as the constituent (b) of the present invention is used to ensure a predetermined hardness and to impart physical strength such as tensile strength to the composition on curing. Examples of suitable inorganic fillers include hydrophilic or hydrophobic fumed silica (dry process silica), precipitated silica (wet process silica), crystalline silica and quartz powder, and these materials may be used singularly, or in combinations of two or more different materials. From the viewpoint of reinforcement, the fumed silica and the precipitated silica should have a BET specific surface area of 50 to 500 m2/g, and preferably 100 to 400 m2/g, and from the viewpoint of factors such as adhesion durability with the fluororesin layer during the formation of the roller, the crystalline silica and the quartz powder should have an average particle diameter of 0.5 to 10 xcexcm, with values from 1 to 5 xcexcm being particularly preferred.
Specific examples of commercially available materials which satisfy these requirements include hydrophilic silica products with brand names such as Aerosil 130, 200, 300 (manufactured by Nippon Aerosil Co., Ltd. and Degussa Co., Ltd.), Cabosil MS-5, MS-7 (manufactured by Cabot Corporation), Rheorosil QS-102, 103 (manufactured by Tokuyama Corporation) and Nipsil LP (manufactured by Nippon Silica Industrial Co., Ltd.), hydrophobic silica products with brand names such as Aerosil R-812, R-812S, R-972, and R-974 (manufactured by Degussa Co., Ltd.), Rheorosil MT-10 (manufactured by Tokuyama Corporation) and the Nipsil SS series of products (manufactured by Nippon Silica Industrial Co., Ltd.), and crystalline silica products with brand names such as Crystallite, Mimusil and Imisil.
The amount of the inorganic filler of the constituent (b) used is typically within a range from 1 to 300 parts by weight of the filler per 100 parts by weight of the constituent (a), with amounts from 5 to 200 parts by weight being particularly preferred. At amounts less than 1 part by weight, the mechanical strength of the silicone rubber deteriorates, and the roller may rupture on removal from the mold. In contrast, at amounts exceeding 300 parts by weight, the compression set of the silicone rubber falls, making it less effective as a roller.
Constituent (c): Platinum Based Catalyst
The platinum based catalyst of the constituent (c) of the present invention promotes the curing addition reaction (hydrosilylation) between the aforementioned constituent (a) and the constituent (d). This platinum based catalyst may utilize any platinum or platinum compound known to promote such addition reactions. Specific examples include platinum black, chloroplatinic acid, alcohol modified products of chloroplatinic acid, and complexes of chloroplatinic acid with olefins, aldehydes, vinyl siloxanes or acetylene alcohol compounds. This catalyst is used in quantities which offer effective catalytic action (so-called catalytic quantity), although the actual amount added may be increased or reduced depending on the curing speed required. Typically, platinum quantities of 0.1 to 1000 ppm are used relative to the amount of the constituent (a), and quantities from 1 to 200 ppm are preferred.
Constituent (d): Organohydrogenpolysiloxane
The organohydrogenpolysiloxane of the constituent (d) of the present invention has the functions of curing the composition and suppressing variations in viscosity. This organohydrogenpolysiloxane has a structure represented by the general formula (2) shown above (namely, a straight chain structure in which the principal chain comprises repeating organohydrogensiloxane units and the two molecular chain terminals are blocked with either triorganosiloxane groups or diorganohydrogensiloxane groups), with at least two silicon atom-bonded hydrogen atoms (in other words, SiH groups) within a single molecule.
In the general formula (2), R3 represents an unsubstituted or a halogen-substituted monovalent hydrocarbon group with no aliphatic unsaturated double bonds, and in the same manner as the group R1 described above, incorporates those monovalent hydrocarbon groups described for the group R of the general formula (1) but excluding alkenyl groups. Specific examples include alkyl groups such as methyl groups, ethyl groups, propyl groups, butyl groups, and pentyl groups; aryl groups such as phenyl groups, tolyl groups and xylyl groups; and halogenated alkyl groups such as 3-chloropropyl groups and 3,3,3-trifluoropropyl groups, although methyl groups are particularly preferred. R4 represents either an unsubstituted or a halogen-substituted monovalent hydrocarbon group with no aliphatic unsaturated double bonds, or a hydrogen atom, and examples of suitable monovalent hydrocarbon groups include the same groups described above for the group R3. X represents an integer of 2 or greater, with integers within a range from 5 to 1000 being preferred. Integers within the range from 5 to 100 are even more desirable.
The viscosity of the constituent (d) at 25xc2x0 C. should preferably be within a range from 3 to 10,000 mPaxc2x7s, with values within the range from 3 to 300 mPaxc2x7s being particularly preferred.
Specific examples of the organohydrogenpolysiloxane of the constituent (d) include methylhydrogenpolysiloxane with both molecular chain terminals blocked with dimethylhydrogensiloxy groups, methylhydrogenpolysiloxane with both molecular chain terminals blocked with phenyldimethylsiloxy groups, and methylhydrogenpolysiloxane with both molecular chain terminals blocked with trimethylsiloxy groups, and these organohydrogenpolysiloxane compounds may be used singularly, or in combinations of two or more compounds.
The constituent (d) is used in a quantity which produces a molar ratio of silicon atom-bonded hydrogen atoms (SiH groups) within the constituent (d) relative to the alkenyl groups bonded to silicon atoms within the constituent (a) within a range from 0.1 to 5, and molar ratios within the range from 0.4 to 3 are particularly desirable. If the molar ratio of silicon atom-bonded hydrogen atoms within the constituent (d) relative to the alkenyl groups within the constituent (a) is less than 0.1, then the cured product may not be sufficiently cured, whereas at molar ratios exceeding 5, increases in viscosity are likely at room temperature, leading to molding difficulties, and there is also an increased danger of gas bubbles forming inside the silicone rubber.
Other Constituents
In order to improve the practical applications of compositions of the present invention, where necessary, various other additives may also be included in such compositions.
In those cases in which the curing time needs to be regulated, retarding agents may be used, and suitable examples include vinyl group containing organopolysiloxanes such as vinylcyclotetrasiloxane, as well as triallylisocyanurate, alkylmaleate, acetylene alcohols and silane or siloxane modified products thereof, hydroperoxides, tetramethylethylenediamine, benzotriazol, and mixtures thereof.
Furthermore, non-reinforcing fillers such as diatomaceous earth and calcium carbonate, coloring agents including organic dyes or inorganic pigments such as cobalt blue, and reagents for improving the heat resistance or flame resistance such as cerium oxide, zinc carbonate, manganese carbonate, titanium oxide and carbon black may also be added.
Roller Production
A silicone rubber roller of the present invention is produced via a series of steps described below.
First, the constituents of a composition of the present invention are either separated into two parts, mixed separately in a static mixer and the two parts then combined, or alternatively all mixed together using the screw rotation action of a dynamic mixer, and the resulting mixture is injected into a roll forming mold with a static metal core which has already undergone primer treatment. The mixture is then cured under heat to form a liquid form silicone rubber covering around the periphery of the metal core, and the roller is then released from the mold. In addition, where necessary a fluorine based latex coating may also be sprayed onto the surface of the released roller, and the coating layer then sintered at high temperature.
In another production method, a roller is formed by first subjecting the internal surface of a tetrafluoroethylene perfluoroalkyl ether (PFA) tube or a polyimide tube to primer treatment, and then injecting an uncured composition of the present invention into the space between a primer treated metal core and the tetrafluoroethylene perfluoroalkyl ether (PFA) tube or polyimide tube, while heat curing is also performed.
In yet another production method, a mixed liquid form addition curing type silicone rubber composition is injected into a roll forming mold with a static metal core which has already undergone primer treatment, the roll forming mold is set in a compression device, and the composition is then cured under heat, a liquid form silicone rubber covering is formed around the periphery of the metal core, and the roller is then released from the mold.
The metal core used in production of the roller may be any one of a variety of materials such as steel, aluminum and stainless steel. Furthermore, a primer treated metal core may also be used. The fluorine based latex coating and the fluorine based resin tubes or polyimide based resin tubes described above may utilize commercially available products, although for the fluorine based resin tubes, tetrafluoroethylene perfluoroalkyl ether (PFA) tubes are preferred, and the surface which contacts the silicone rubber should preferably be treated by either corona discharge or by other treatments which improve the contact with silicone rubber such as sodium naphthalene methods, sputter etching methods or liquid ammonia methods. In addition, primer treatment may also be used to improve the contact durability.